Alternative: click the Add Media button of the post editor, click the Gallery tab, click Show next to the image you want to edit, enclose the word you want to change in the following code then scroll down and click “Save all changes”: <span style="color:#123456;">WORD HERE</span>
(Replace 123456 with the hex for the color you need.)
Repeat for the other images.
I just tried this technique to change a white colored caption to black in one of my slideshows. I used #000000 for the color ‘black’. I apparently goofed up somewhere because after I saved the change, the caption included the whole line of code.